A new method is described for simultaneously monitoring compliance with prescribed-diet and urine-collection regimens during metabolic balance studies. It involves incorporation of the potassium salt of p-aminobenzoic acid (PABA) into provided foods and measurement of its extent of recovery in urine. A validation study was conducted in 10 healthy adults. During three consecutive 24-h periods, subjects consumed four foods per day that had each been supplemented with 72.0 mg PABA and also made complete urine collections. An additional 24-h urine collection was made by each subject for determination of baseline PABA excretion. The PABA was easily incorporated into the test foods and was not detected by taste. The recovery of the PABA in urine, determined by colorimetric assay, was 98.7 +/- 3.7% (means +/- SD) of intake. Thus, in metabolic studies for greater than or equal to 3 d, PABA can be used to provide a sensitive index of dietary compliance.